Method for retreading tires

ABSTRACT

The specification discloses a method for retreading tires with continuous precured and premolded tread strips. A method of adapting a standard tire retreading mold to use the precured tread strip is also disclosed. The method makes it possible to adapt a single size retreading apparatus or existing mold to retread a number of different sized tires by using a retreading chamber larger than the tire, a flexible envelope around the exterior of the tire, and an inflatable means to expand the sidewalls of the tire laterally outward to engage sealing means mounted on the inner sidewalls of the retreading apparatus. In addition, vent means are provided to remove air trapped between the envelope and the tire.

i 1 METHOD FOR RETREADING TIRES [75] Inventor: Willi E. Reppel, Lewiston, Maine [73] Assignee: Oliver Tire & Rubber Company,

' Oakland, Calif.

[22] Filed: Sept. 22, 1971 21] App1.No.: 182,814

Related US. Application Data [62] Division of Ser. No. 883,161, Dec. 8, 1969,

abandoned,-

[52] US. Cl .Q 156/96, 156/394 FM, 264/36,

264/315, 264/316, 425/19, 425/39, 425/43, 425745 [51] Int. Cl B29h 5/04,B29h 5/18 [58] Field of Search 264/36, 313,314, 264/315, 316; 425/ D1G. 19, 39, 43, 44, 45; 156/96, 394

[56] References Cited UNlTED STATES PATENTS 2,014,010 9/1935 Wheatley 156/394 FM 2,904,842 9/1959 Alm 156/394 3,207,647 9/1965 Schelkmann 156/394 2,459,757 1/1949 Smith 156/96 X 2,501,577 3/1950 Pleiffer 156/394 X 2,746,515 5/1956 Usack A 156/116 2,774,409 12/1956 Skidmore 156/96 3,486,198 12/1969 Lewis 425/43 Primary EraminerAlfrcd L. Lcavitt Assistant Examiner-C. B. Cosby Alt0rneyK. 1.. King [57] ABSTRACT The specification discloses a method for retreading tires with continuous precured and premolded tread strips. A method of adapting a standard tire retreading mold to use the precured tread strip is also disclosed.

14 Claims, 10 Drawing Figures PATENIEDnEc 1a 1915 SHEEY 10F 5 FIG: 1

3| (l5b K3:, I

PATENTED DEC 1 8 I975 SH'iET 2 BF 5 METHOD FOR RETREADING TIRES This is a division of application, Ser. No. 883,161, filed Dec. 8, 1969, now abandoned. This divisional application is copending with U.S. Ser. No. 182,602, and U.S. Ser. No. 182,603, both of which are divisional applications of the parent U.S. application Ser. No. 883,151, now abandoned. This application is also copending with U.S. Ser. No. 247,488, a continuation of U.S. Ser. No. 883,161.

BRIEF SUMMARY OF THE INVENTION to the old casing by a combination of heat and pressure.

The problem with retreading a tire in a standard retreading mold resides in the fact that the finished tread will quickly wear in use. This is because the layer of uncured rubber cannot be subjected to extremely high molding pressures and temperatures while being vulcanized without permanently deforming or distorting the casing. Any deformation or distortion during the curing operation will subsequently tend to rupture or weaken the vulcanized bond between the cured and molded tire tread and the carcass itself. Any excessive temperature also tends to weaken and deteriorate the tire carcass which was previously cured.

Moldless retreading, on the other hand, uses a strip of precured and premolded tread which is usually sup plied in a long, continuous strip from an extrusion and molding process. This precured and preformed strip of tread material is, in a sense, vulcanized before it is applied to the tire casing, as considerably higher vulcanization pressures and temperatures may be applied while the strip is being molded without the the carcass. This results in a tread surface which will have improved strength and density and which will resist normal road abrasion.

In this method, the worn-out tire is treated by a buffing machine as in the molding operation, and the precured and premolded tread rubber is applied to the buffed casing with a layer of uncured raw rubber compound therebetween. The tire is then wrapped up in an adjustable thin steel band and inflated and the entire assembly is heated and cured in a pressurized and heated chamber.

An alternate form of moldless retreading with the tread strips that are precured and premolded utilizes a flexible rubber envelope that continously covers the outside of the tire when the tire is mounted on a-tire rim. The entire assembly is again inserted into a pressurized and heated chamber, but the envelope gives a uniform pressure to the tread and casing and assists in giving a uniform and continuous bond while assisting in the removal of any possible air bubbles.

This method'of moldless retreading requires the operator to invest in expensive equipment to utilize the new premolded and precured tread, whereas in the present invention, the operator may adapt his older retreading molds to apply the new preformed and precast tread.

The present invention makes it possible to utilize the precured and premolded tread that has become popular in an existing standard tire retreading mold. In addition, the invention allows retreading of various sized tires within a single size tire retreading mold. The present invention utilizes a mold liner to seal the interior of the mold and a gasket means to seal various sized tires within the mold.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial cross section of a standard tire retreading mold, as fitted for use in retreading a tire with a precured and precast tread;

FIG. 2 is an enlarged partial cross section of a standard tire retreading mold, and the mold liner used therein;

FIG. 3 is a plan view of the gasket sealing means used in the present invention;

FIG. 4 is an isometric view of the tire assembly prepared for retreading as it is inserted into a standard tire retreading mold;

FIG. 5 is an exploded cross-sectional view of the entire assembly and apparatus;

FIG. 6 is a partial cross-sectional view of a new and improved adjustable mold, as adapted for retreading of various sized tires;

FIG. 7 is a cross sectional view of a new and improved mold;

FIG. 8 isa layered, sectional, and isometric view of a tire assembly prepared for retreading, an air bleeding means, and the inner mold liner; and

FIG. 9 is a cross-sectional view of a new and improved mold for retreading tires.

FIG. 10 is a cross sectional view of a retreading apparatus and a tire assembly prepared for retreading, wherein said assembly is mounted on a rim.

DETAILED DESCRIPTION OF THE DRAWINGS Referring to FIG. 1, a worn-out tire casing 11 has been prepared for retreading by buffing it on a buffing machine to remove any of the old tread design. The buffing not only cleans the surface of the casing but roughens it in preparationfor the bonding and vulcanizing processes to follow. A liberal coating ofa solvent or rubber cement is applied to the roughened casing. Another layer of solvent or rubber cement is applied to the under surface of the premolded and precured tread strip 13. The underside of this tread strip is also buffed or manufactured with a pre-roughened surface. The rubber cement or solvent is utilized to hold the tire assembly prepared for retreading together before it is placed in the tire retreading mold. A curing bag 14 is placed within the tire and partially inflated. The entire assembly is then enveloped with a nylon mesh air bleeding means 61, and an inner envelope 60. This assembly is then placed within a standard tire retreading mold 15 which has been lined with mold liner l6, and a seal is affected between the mold liner 16 and envelope 60 by means of gasket members 17 and 18.

Pressure is applied to the inside and outside of the tire casing by inflating the curing bag 14 with hot air or steam through nozzel 19.Simultaneously, air pressure or hydrostatic pressure is applied to the chamber 20 maintained between the tire prepared for retreading and the mold liner by means of valve member 21 and- /or 22. Pressure in the curing bag and in chamber may vary between 20 and 200 pounds per square inch. A lower pressure will extend the lifetime of the curing bag, envelope and mold liner, but it may cause bad adhesion between the components of the tire prepared for retreading. The mold liner itself is suitable for maintaining a pressure of 200 pounds per square inch, and the curing bag 14 is normally maintained at a pressure higher than that maintained in chamber 20 by the standard inflation difference that is normally maintained in the tire. In one typical example, if the tire is designed to operate at pounds of pressure in normal use, chamber 20 will be inflated to 75 pounds per square inch and the curing bag to approximately 100 pounds per square inch. Thus, there is an effective adhesion pressure of 75 pounds per square inch forcing the tread layer 13 towards carcass ll.

Alternately, as illustrated in FIG. 10, the tire may be.

mounted on a rim 170 and inflated with a standard, inner tube 171 before it is placed in the mold. In this application, however, the air pressures utilized may be somewhat reduced.

The temperatures utilized typically range between 150F. to 300F., depending upon the technical properties of the used casing. Generally speaking, a high temperature makes for a short curing time but may impair the quality of the casing. A low temperature does not generally affect the tire casing itself but does unduly extend the curing time. The temperature is generated by supplying 300 pressurized air or steam to curing bag 14 through nozzel 19. High temperature at the exterior of the casing will result from circulating 300 air or steam in chamber 20 by introducing it through inlet 21 and exhausting it through inlet 22. Additionally, the heating apparatus in the standard tire retreading mold 15 may be utilized, and steam or hot air will be circulated through passages 23 and 24 or as in the case of an electric mold, the heating element of the standard tire retreading apparatus may be energized.

The tire retreading is finished when the uncured layer of rubber 12 is vulcanized and fully bonded to casing 11 and tread member 13. This vulcanization process completes the cure of tread member 13, if it is a partially cured tread, but in any event vulcanizes the tread, adhesion layer, and carcass into a unitary assembly.

Referring to FIG. 5, the mold liner consists of a plurality of layers. The first layer 25 extends over the tire assembly prepared for retreading from sidewall to sidewall to form an effective seal between the mold liner and the standard tire retreading mold. A second and third layer 26 and 27 are applied on top of the first layer to cover the tread molding area generally desig nated at on mold 15. A fourth layer 28 is applied over the top of the two layers 26 and 27 to cover them and bond the four layers into a single mold assembly. Layers 25 and 28 are formed of a thin flexible nonthermoplastic material; whereas layers 26 and 27 are formed of relatively thick uncured layers of raw rubber. Short lengths of steel bands or cords 29 may be inserted between layers 26 and 27 to strengthen and provide some structured rigidity for the mold liner.

It will be noted in FIG. 2 that the mold 15 is split into two sections 15a and 15b. Similarly, the mold liner 16 is split at 32 and gasket 33 is applied to one side of the mold liner. Layer 33 is a soft cured flexible layer that seals against the opposing section of the mold liner to prevent passage of any fluids therebetween.

There are two problems inherent in utilizing the mold liner which must be balanced against each other to achieve satisfactory operation. The first problem is transferring the heat that may be generated in mold 15 through the mold liner and into the interior chamber 20. The other problem is the reusability of the mold liner, and in providing for its ease of insertion. If a relatively hard layer were used adjacent the tread pattern 30, very little heat would be transferred because it would cover the actual tread channels and ridges that normally provide a much larger heat transfer surface. In such case, the only heat transfer would be across the general uppermost area of the tread pattern.

If a soft flexible material is utilized, the material will fill the tread pattern completely, filling the voids such as indicated as 30a and will be cured in that position by the temperature and pressure of the vulcanized process. Although the soft material will utilize all of the heat transfer surface, it then becomes extremely difficult to separate the mold liner from the mold casing 15, and almost impossible to reinsert the mold liner for future use. Therefore, the present multi-layered mold liner has been found to be an equitable balance to solve the two problems. Utilizing the relatively thick, soft uncured rubber layers 26 and 27, the rubber will migrate into the tread pattern 30 as illustrated in FIG. 2, thereby increasing the effective heat transfer surface of the mold liner. Using a relatively thin, non-thermal plastic layer 28 which is already cured, prevents the relatively soft layers 26 and 27 from migrating into void 30a, and enables the operator to remove the mold liner after the curing operation is completed, and to insert it for future use with a minimum amount of effort. The inserted mold liner halves normally stay in the proper mold half if the operator retreads several tires in the same apparatus. They are removed only if mold is reutilized for conventional retreading.

The mold liner is necessary to effectively seal air passages 35 that are utilized in the standard retreading mold to allow the raw uncured rubber to migrate into the tread pattern 30.

' An interior envelope is shown in cross section in FIG. 8, draped about carcass 11 and tread cap 13. This interior envelope is placed about the tire assembly prepared for retreading before it is placed in the mold. Sandwiched between the tire assembly prepared for retreading and the envelope 60 is air bleeding means 61. This air bleeding means may be formed of a woven-mesh cloth such as a nylon mesh or the like and allows the air trapped between the tire assembly prepared for retreading and the inner envelope to migrate from the trapped cavities to the ambient air pressure at 62. Inasmuch as the gasket and sealing means generally contact the inner envelope at the dotted line illustrated at 63 the air bleeding means 61 effectively provides an air passageway from the cavity between the interior envelope and the tire assembly to the outside atmosphere. In this way, any air trapped in the tire assembly or between the tire assembly and the envelope liner is expelled when the pressure in chamber 20 is raised above the ambient atmospheric air pressure.

The purpose of the inner envelope 60 is two-fold. First, it enables the air pressure to be applied uniformly over the tread-capped surface throughout the curing process. If the tire were placed in the tire retreading assembly without the envelope, the air present between layers 11 and 13, when subjected to the heat, would expand causing the tread layer 13 to lift from the carcass. Once the tread cap lifts from the carcass, without the envelope, the air in chamber 20 at that pressure enters the joint between tire carcass l1 and tread cap 13 and provides equal pressure on both sides of tread cap 13, thereby assisting the natural resiliency of the tread cap and allowing it to become separated from the tire carcass. In this manner, the inner envelope 16 not only distributes pressure evenly from the cap 'to the carcass but also prevents any of the high pressure air present in chamber 20 from entering into the sealing'region between the carcass 11 and tread cap 13 to thereby destroy the bond.

The second purpose of the interior envelope is to provide an effective escape route through the nylon mesh 61 for any air that is trapped in the seal-between air carcass 11 and the tread member 13.

Referring again to FIG. 5, it will be noted that gasket members 17 and 18 illustrate an alternate construction from that illustrated in FIG. 1. The present invention enables the owner of a standard tire retreading apparatus illustrated as to utilize the newer premolded and precured tire tread in retreading tires. It also enables the operator to retread a variety of different sizes of tires by use of spacer members 36 and 37 illustrated on gaskets 17 and 18. Thus, an operator may adapt his mold to as many as four cross sectional sizes below the cross sectional size of the tire mold. The largest size tire that may be utilized with the mold liner would be one size below the mold size. A plurality of bead sizes may also be used, i.e., 6.50-l4; 6.70-l4; 7.00-l4, so that in actuality if three cross sectional sizes and four bead sizes are used, tht total number of combinations would be twelve. Spacer members 36 and 37 enable the operator to utilize a plurality of bead sizes by inserting the appropriately sized bead spacer as illustrated in FIG. 3. The gasket sealing means 17 comprises spacer member 36, annular support member 38, inlet means 21, exhaust means 39, and means for providing an air pressure reading of the interior chamber 40. In actuality, the functions of 21, 39 and 40 are interchangeable, with the inlet and exhaust being supplied to either and the pressure gauge being supplied to any one of the three. The end of inlet, pointing into chamber 20 may be provided with a jet, injecting air or steam in a tangential direction into chamber 20for better circulation. This prevents any localized hot spots that might otherwise distort the carcass and result in an uneven cure. Alternately, only one such gasket means such as 17 need be used, as the tire sidewall 41 illustrated at FIG. 1 will form an effective seal between layer 25 and wall member 42 without the necessity of using gasket member 18.

When two gasket sealing members such as 17 and 18 are utilized, the inlets 21, 39 and 40 and gasket member 17 will be used as supply, and the inlets generally designated as 22 in gasket member 18 will be used as exhaust, with one of the inlet members or exhaust members being utilized as a pressure reading station.

FIG. 4 is an isometric view of a tire assembly prepared for retreading about to beinserted into a standard tire retreading mold apparatus. Mold liner 16 is shown inserted in the illustrated mold half. The tread pattern 43 illustrated on flexibleenvelope results from previous application of heat and pressure. The two halves of mold liner 16 are joined together along seam 32 as the other half of the tire retreading mold illustrated as 15b in FIG. 5 (not shown in FIG. 4) is closed and latched to tire mold apparatus 15a.

A new and improved form of the present invention is illustrated in FIGS. 6 and 7, wherein a treadless mold is utilized with a single layer flexible envelope 160 to retread a tire carcass 111 with a premolded and precast tread member 113. It will be noted that the envelope 160 now forms chamber between the mold 115 and envelope 160 rather than between envelope 160 and tire carcass 111. Gasket means provides an effective seal for mold 115 between the respective halves 115a and llSb to provide an airtight chamber 120. Passageways 123 and 124 are for the circulation of hot air, steam or other type temperature fluids, or an an alternative embodiment, passageways 123 and 124 may be replaced with electric heating means. A curing bag 114 is utilized in this new and improved embodiment in the same manner in which it was utilized in a standard tire retreading apparatus. Alternatively, as illustrated in FIG. 10, a tire mounted on a rim with a conventional inner tube could be used in place of the curing bag. Gasket members 117 and 118 may be simplifled somewhat in that air passageways 121 and 122 are now vented through the wall of chamber 115 rather than through the gasket members 117 and 118. Recesses 151 and 152 are provided in the interior wall of mold 115 through sealing gasket members 117 and 118.

It will be noted that FIG. 6 is a partial cross section of the new and improved mold of the present invention, whereas FIG. 7 is a full cross section. Removing the tread pattern or tread base from the interior of mold 115 has two primary advantages: first, it eliminates the somewhat complex mold liner required for use in the standard tire retreading mold and secondly, it allows an additional cross sectional size of tire to be used. In the apparatus disclosed in FIGS. 6 and 7 four cross sectional sizes may be used with a number of bead sizes whereas in a standard tire retreading mold, only three cross sectional sizes may be used. The number of bead sizes that may be adapted remain the same for each type of apparatus. In FIG. 7, it will be noted that mold 115 has access holes 153 and 154 to assist in the assertion of the circulating fluids to nozzle 119 or curing bag 114 and to assist the utilization of the standard mold loading equipment used to load and center the tire assembly prepared for retreading in the mold. In actual practice, an appropriately sized gasket and spacer 136 is utilized to adapt each of the various sized bead sizes to the mold liner.

The outer parts 155 of spacer 136 and the outer part 55 of spacer 36 in FIG. 5 form annular ridges to assist in providing an airtight seal between spacer 136 and envelope or between spacer 36 and tire carcass 11. The pressure for sealing chamber 120 is provided by inflation of curing bag 114, pressing tire sidewall 111 and envelope 116 against spacer 136 as well as outer part 155. This forces the tire against portion 155 to effect an air tight seal.

7 FIG. 9 illustrates another alternate and improved embodiment of my invention. It has been found that this embodiment is much easier, simpler and more inexpensive to manufacture than a standard retreading mold, and is readily adaptable to various sized tire carcasses. The base member 215 is a flat annular disk member provided with strengthening ribs (not shown) to adequately disperse the tensions between the central support 266 and the outer rim noted at 267. Another flat annular disk member 268 is provided with four (4) stiffening rims such as those illustrated at 269, 270 and 271. The third component of my new and improved tire retreading assembly is annular ring member 272 which is provided with air inlet means 273 and air outlet means 274. Chamber 220 is sealed by means of a flat annular gasket member 275 which seals not only the annular ring member 272, but also the tire side wall as shown at 216. An alternate sealing means is illustrated at 276 wherein an annular O-ring is provided in the outer extremity of ring member 272 to engage disk member 268 in a sealing arrangement.

With this embodiment, the operator may retread four or live tires of different cross sectional sizes with a single annular ring member 272. If on the other hand, the operator desires to retread a different cross-sectional size, such as 6.50 rather than 8.50, he needs only replace the annular ring member 272. In this manner it then becomes unnecessary to have more than one retreading apparatus. With my present invention, the only things needed to encompass an entire range of tire sizes are disk members 215 and 268 and a plurality of annular ring members 272 in different sizes.

It should be pointed out that sealing means 276 is an alternate form of the invention and may be utilized on both sides of ring member 272, or in the alternative, the annular gasket member 216 could be utilized on both support members, 215 and 268 to provide for effective gasket sealing on both sides for both the tire assembly prepared for retreading and the annular ring member 272.

Lug nut 277 is used to tighten the assembly and to provide effective sealing pressure against the seal gasket members 276 and 275 and thereby provide an airtight chamber 220.

In the operation of my device, lug nut 277 is removed and the upper disk portion 268 is removed. The tire assembly prepared for retreading, and its inner envelope 60 and air bleeding means 61 are placed into the tire assembly as shown in FIG. 9 and approximately centered therein. The upper disk portion 268 is then replaced and the entire assembly tightened with lug nut 277 to provide an effective air-tight seal in chamber 220. A source of high pressure, high temperature fluid such as steam is connected to fluid inlet 273 and 219 to provide an effective sealing differential pressure between chamber 220 and curing bag 214. lt will be noted that air inlet 273 is angled in a circumferential direction, so as to cause the steam introduced into chamber 220 to circulate throughout the chamber in a circumferential direction before exiting at fluid exit 274. This prevents the steam inlet from focusing on any small portion of the tire assembly prepared for retreading and thereby prevents any uneven curing that might result therefrom. As pointed out previously, any air trapped between the tire carcass 211 and tread member 213 is effectively released by means of bleeding means 261 which is placed between inner envelope 260 and tire assembly 211. Inasmuch as the nylon mesh member 261 communicates with the ambient outside air pressure at 280, the differential pressure between chamber 220 and the outside air pressure will effectively drive any trapped air therein to the outside ambient air.

It is to be understood that, although several preferred embodiments of the invention have been shown in the drawings and described with considerable particularity in the foregoing specification, the invention is not limited to the specific details and forms shown and described, but includes all modifications coming within the scope of the invented claims and their equivalents.

I claim:

1. A process for retreading a tire with a precured, promolded tread strip comprising:

a. covering a tire and precured tread assembly with a thin flexible envelope which extends from sidewall to sidewall around the outer perimeter of said tire,

b. inserting inflatable means into the interior of said assembly,

c. inserting said assembly into a multipart rigid shell having a generally annular shape with a U-shaped cross-section, said shell defining a sealed chamber around the thread portion of the assembly,

(1. sealing said chamber by urging said assembly and the flexible envelope into contact with sealing means defined on the inner walls of said shell adjacent each of the assembly sidewalls, said sealing means cooperating with the inner walls of the shell and the exterior surface of the envelope to provide a fluid tight seal for a chamber defined between said shell and said envelope,

e. inflating said inflatable means to laterally displace the sidewalls of said assembly and urge them into contact with said sealing means,

f. providing heated and pressurized fluid within said annular chamber defined between said shell and said envelope, the pressure in said chamber being lower than the pressure in said inflatable means,

g. simultaneously evacuating air trapped between the surface of the tire assembly and the envelope through a means inserted between said envelope and said assembly.

2. A process as claimed in claim 1 wherein said tire assembly and said envelope are inserted into a standard tire retreading mold, said mold having been previously lined with a mold liner to seal the interior of said mold.

3. A process for retreading a tire as claimed in claim 1 wherein the pressure maintained in said inflatable means is at least 20 pounds higher than the pressure maintained in the sealed chamber.

4. A process for retreading a tire as claimed in claim 1 wherein said tire assembly is mounted on a rim prior to its insertion in said multipart shell.

5. A process for retreading a tire as claimed in claim 1 wherein said fluid comprises steam having a temperature of 2l2 300.

6. A process for retreading a tire with a precured, premolded tire tread comprising:

a. preparing a tire casing for retreading by roughing the exterior tread receiving portion;

b. applying a precured tread strip to the exterior of the tire casing with an uncured layer of rubber I therebetween to form a tire assembly prepared for retreading,

0. covering said tire assembly with a thin, flexible envelope which covers said tread strip and extends from sidewall to sidewall around the outer perimeter of said assembly;

d. inserting an inflatable means into the interior of said casing,

e. inserting said tire assembly and said envelope into a multipart rigid shell having a generally annular shape and a U-shaped cross-section, said shell defining a sealed chamber above the tread portion of the assembly,

f. sealing said mold to define a fluid-tight chamber between said mold and the exterior of said envelope, said sealing also including the pressurization of said inflatable means to laterally distend the sidewalls of the easing into contact with the sealing means mounted on the inner walls of the shell adjacent the casing sidewalls,

g. pressurizing both the fluid-tight chamber and the tire assembly to provide positive fluid pressures therein with a higher fluid pressure in the tire assembly;

h. simultaneously exhausting any air trapped between said flexible envelope and said tire assembly with venting means located between said envelope and said casing,

. heating said tire assembly within said mold to bond said strip to said casing.

7. A process as claimed in claim 6 wherein said tire assembly and said envelope are inserted into a standard tire retreading mold, said mold having been previously lined with a mold liner to seal the interior of the mold.

8. A process for retreading a tire as claimed in claim 6 wherein the pressure maintained in said inflatable means is at least pounds higher than the pressure maintained in the sealed chamber.

9. A process for retreading a tire asclaimed in claim 10 I 6 wherein said tire assembly is mounted on a rim prior to its insertion in said multi-part shell.

10. A process for retreading a tire as claimed in claim 1 wherein said assembly is heated by a fluid having a temperature of at least 11. A process for retreading a tire as claimed in claim 10 wherein said fluid is steam having a temperature of 212 300.

12. A process for retreading a tire as claimed in claim 1 wherein said sealing means is inserted into the sidewalls of said shell before said shell is sealed.

13. A process for retreading a tire as claimed in claim 12 wherein said shell is adjusted to various sized tires by inserting differentially sized sealing means to match the tire to the shell.

14. A process for retreading tires as claimed in claim 6 which further comprises a. inserting said enveloped tire assembly into a retreading assembly having a flat circular base portion, upwardly extending securing means, and an upwardly extending thin annular ring that circumferentially surrounds the tire assembly,

b. covering said tire assembly and said annular ring with a cover means and c. securing said cover means to said base by said upwardly extending securing means to define a sealed chamber therein. 

2. A process as claimed in claim 1 wherein said tire assembly and said envelope are inserted into a standard tire retreading mold, said mold having been previously lined with a mold liner to seal the interior of said mold.
 3. A process for retreading a tire as claimed in claim 1 wherein the pressure maintained in said inflatable means is at least 20 pounds higher than the pressure maiNtained in the sealed chamber.
 4. A process for retreading a tire as claimed in claim 1 wherein said tire assembly is mounted on a rim prior to its insertion in said multipart shell.
 5. A process for retreading a tire as claimed in claim 1 wherein said fluid comprises steam having a temperature of 212*- 300*.
 6. A process for retreading a tire with a precured, premolded tire tread comprising: a. preparing a tire casing for retreading by roughing the exterior tread receiving portion; b. applying a precured tread strip to the exterior of the tire casing with an uncured layer of rubber therebetween to form a tire assembly prepared for retreading, c. covering said tire assembly with a thin, flexible envelope which covers said tread strip and extends from sidewall to sidewall around the outer perimeter of said assembly; d. inserting an inflatable means into the interior of said casing, e. inserting said tire assembly and said envelope into a multipart rigid shell having a generally annular shape and a U-shaped cross-section, said shell defining a sealed chamber above the tread portion of the assembly, f. sealing said mold to define a fluid-tight chamber between said mold and the exterior of said envelope, said sealing also including the pressurization of said inflatable means to laterally distend the sidewalls of the casing into contact with the sealing means mounted on the inner walls of the shell adjacent the casing sidewalls, g. pressurizing both the fluid-tight chamber and the tire assembly to provide positive fluid pressures therein with a higher fluid pressure in the tire assembly; h. simultaneously exhausting any air trapped between said flexible envelope and said tire assembly with venting means located between said envelope and said casing, i. heating said tire assembly within said mold to bond said strip to said casing.
 7. A process as claimed in claim 6 wherein said tire assembly and said envelope are inserted into a standard tire retreading mold, said mold having been previously lined with a mold liner to seal the interior of the mold.
 8. A process for retreading a tire as claimed in claim 6 wherein the pressure maintained in said inflatable means is at least 20 pounds higher than the pressure maintained in the sealed chamber.
 9. A process for retreading a tire as claimed in claim 6 wherein said tire assembly is mounted on a rim prior to its insertion in said multi-part shell.
 10. A process for retreading a tire as claimed in claim 1 wherein said assembly is heated by a fluid having a temperature of at least 150*.
 11. A process for retreading a tire as claimed in claim 10 wherein said fluid is steam having a temperature of 212*- 300*.
 12. A process for retreading a tire as claimed in claim 1 wherein said sealing means is inserted into the sidewalls of said shell before said shell is sealed.
 13. A process for retreading a tire as claimed in claim 12 wherein said shell is adjusted to various sized tires by inserting differentially sized sealing means to match the tire to the shell.
 14. A process for retreading tires as claimed in claim 6 which further comprises a. inserting said enveloped tire assembly into a retreading assembly having a flat circular base portion, upwardly extending securing means, and an upwardly extending thin annular ring that circumferentially surrounds the tire assembly, b. covering said tire assembly and said annular ring with a cover means and c. securing said cover means to said base by said upwardly extending securing means to define a sealed chamber therein. 